1. Field of the Invention
The present invention relates to an image processing system. In particular, the present invention relates to an image processing system that can prevent an image from being tampered by embedding a digital watermark. Here, the tamper means the operation of alteration or replacement. The alteration changes the value of the data, which constitute the contents of the image. The replacement replaces the image to other data.
2. Description of the Related Art
There exist some needs for managing documents as image data, such as billing or recording sheets, and utilizing the electric documents in a network-connected image processing system. The image processing system has to process the large amount of the images of the documents having various document types. The image processing system also has to be strictly secure from tampering with the electric documents.
The conventional image processing system manages the image basically by controlling the access to the image storing and processing apparatus. The method of controlling the access prevents the image file from being tampered by prohibiting writing to the image file and permitting reading the image file. The access control also can be realized by a system coupling with a cryptosystem. The system encodes the image and stores the decoding key and the encoded image. The encoded image cannot be decoded without the decoding key, and the image cannot be processed without decoding the encoded image.
As a method of detecting tampering with the image other than encoding the image itself, there is a method which generates a characteristic bit pattern such as a message digest, a compression parameter, or a hash value, which function like the fingerprint of the data, from the image data as a verifying data. The method then manages the bit pattern independently of the image, generates the verifying data from the image to be verified, and compares the generated verifying data with the managed verifying data. If the generated verifying data is different from the managed verifying data, it can be recognized that the image data has been tampered. The message digest function to generate the verifying data, which is most generally used, is a series of the algorithms such as MD2, MD4, and MD5 by the RSA Security Inc.
Even in the system which generates the verifying data and detecting tamper by examining the verifying data, both the image and the verifying data can be replaced by generating the verifying data from the tampered image, and thus the verifying data must be encoded before using.
As a cryptosystem, generally, there are a “common key system”, which encoding and decoding the data by a common private key, and a “public key system”, which uses a public key that is open to the public and a private key that is secret to the people other than possessor of the key. The data is encoded by the public key and decoded by the private key in the public key system. In addition, a digital signature is used for the authentication. As an example of the common key system, there are the DES system, Data Encryption Standard system, the FEAL system, Fast Encryption Algorithm system by Nippon Telegraph and Telephone Corporation, the MISTY system by Mitsubishi Electric Corporation. As an example of the public key system, there is the RSA public key system by RSA Security Inc.
As shown in Nikkei Electronics, Feb. 24, 1997 (no. 683), the conventional digital watermark technology was mostly used for copyright protection. Many methods have been proposed where the image quality does not deteriorate and the watermark is embedded as inerasably as possible. These methods use digital watermark, which is difficult to recognize and difficult to erase even by alternating the image, to embed an information and confirm the copyright by examining the embedded information. Furthermore, the digital watermark was embedded all over the image because the digital watermark was used for protecting the whole image.
As a format identifying technology of the image, there is a method of identifying the format by analyzing the structure of the frame format in the form as disclosed in Japanese Patent Application Laid-Open No. H4-158487. There is also a method of distinguishing an image quality and separates into regions as described in the Journal of the Institute of Image Electronics Engineers of Japan in the 124th issue, Vol. 25, No. 5, P. 533-P. 540, 1996.
As a method of using the digital watermark for the image verification, it is proposed to embed a stamping information in the original image by using a steganography, or data hiding, that is one characteristic of the digital watermark. Tamper can be detected by extracting the stamping information from the original image and examining the stamping information. Many uses are proposed for information embedding using the digital watermark. The systems are proposed that embed various managing informations such as an information of apparatus that generates an image, creator information, and user information other than the copyright information, in image by the digital watermark.
However, tampering by a privileged user cannot be prevented by operating the access right only. It cost a lot to configure a complete set of the access rights for all apparatus that process the images. The image managing method by encoding needs to decode or re-encode an image file to display the image because the image file itself is encoded. Therefore, it causes a problem that the image processing load increases, especially for the system which processes a large amount of images.
In a system that uses the images in a plurality of machines distributed within the system, it is difficult to manage perfectly the encoding and decoding processes, the encoding and decoding keys, and the decoded images, because the decoding or the encoding is processed at each machines. Therefore, iniquity can be easily done by attacking a machine, which is less strictly secured. In the case of detecting tamper by a verifying data such as a message digest, the cryptosystem that manages the verifying data is needed to protect the verifying data when managing the verifying data, and the same problems may occur as the image encoding systems described above.
Furthermore, the method of using the digital watermark in the conventional image processing system described above embeds the digital watermark all over the image because the purpose of using digital watermark is to protect the whole image. There was no problem when using the digital watermark for the purpose of protecting the copyright of the image such as a single image of a picture or a photograph.
However, embedding the digital watermark increases the quantity of information of the image itself because some additional information are inserted into the image as noise components, although many methods have been proposed for minimizing the visual influence on the image. If the image is not compressed the size of the image, in which the digital watermark is embedded, is the same as the size of the original image. If the image is compressed, in most cases, the size of the image, in which the digital watermark is embedded, is larger than the size of the original image so that a large storing space will be needed and costs lots. It also takes time to process the image.
Additionally, there is a possibility of decreasing the image quality depending on the location when the digital watermark is used for an image having images with different qualities such as a character and a photograph. It is because the digital watermark technology does not take the difference in the image quality. There is a case that the image cannot be recognized because the conventional use of the digital watermark does not consider the influence on the character recognition of the image by OCR, so that the embedded watermark can be a noise for the character recognition.